1. Field of the Invention
This invention relates generally to slip-casting of ceramic articles of manufacture and pressure casting of metal articles of manufacture and more specifically to an improved ceramic shell casting system involving successive wet dipping and dry powder coating, or stuccoing, to produce a multi-layer shell mold for these applications.
2. Description of the Related Art
Aerothermodynamic experimentation of modern fluid dynamic principles in the aerospace industry requires among other things high precision and detail in the fabrication of test models. Scientists and engineers have employed test models fabricated from ceramic materials because they exhibit low heat conductivity; possess relatively low coefficients of thermal expansion (CTE); and are capable of surviving high testing temperatures.
Heretofore, ceramic test models were fabricated either by machining or by casting. Cast models can be replicated easily and inexpensively, thus casting processes are preferable to machining processes.
Slip-casting methods provide superior surface quality, density and uniformity in casting high-purity ceramic materials over other ceramic casting techniques, such as hydraulic casting, since the cast part is a higher concentration of pure ceramic powder with little additives. Ceramic powder is compacted in the slip casting process and sintered or bonded together at high furnace temperatures. A slip is a crowded suspension of fine ceramic powder in a liquid such as water or alcohol with small amounts of secondary materials such as dispersants, surfactants and binders. Early slip casting techniques employed a plaster-of-Paris block or flask mold. The plaster-of-Paris mold draws water from the poured slip to compact and form the casting at the mold surface. This forms a dense cast form removing deleterious air gaps and minimizing shrinkage in the final sintering process.
U.S. Pat. No. 5,266,252 ("the '252 Patent") observed that previous slip casting techniques often did not provide the degree of detail necessary for some wind tunnel testing. Indeed, detailed parts were frequently marred with parting lines or suffered surface damage when removed from the stiff flask mold. The '252 Patent resolved this problem by employing a calcium sulfate bonded refractory mix in a quick setting slurry to form a "shell" mold. The primary advantages of which were the ability to use the lost wax process for net casting and seamless molds. The disadvantages have been that these molds, as well as the plaster of Paris molds, are dimensionally unstable during set-up and that the mold forming process is difficult to automate.
U.S. Pat. No. 4,865,114 ("the '114 Patent") also made use of the shell mold using the calcium sulfate bonded refractory mix in a quick setting slurry for pressure casting of non-ferrous metals. This molten metal casting process used pressure and the high density shell mold to form parts with small channels and high surface detail.
Ceramic shell casting techniques using silica, zirconia and other refractory materials are currently used by the metal parts industry for `net casting,` forming precision shell molds for molten metal casting. The technique involves a successive wet dipping and dry powder coating or stucco to build up the mold shell layer. The shell casting method in general is known for dimensional stability and is used in many net-casting processes for aerospace and other industries in molten metal casting. Automated facilities use multiple wax patterns on trees, large slurry mixers and fluidic powder beds for automated dipping.
Such a shell casting system would be useful for precision and automation in slip casting, except with current shell casting systems the fired strength is too high for removal from delicate slip-cast ceramic parts and can be troublesome for core removal in even molten metal castings. In addition, current shell casting systems do not draw liquid, or `slip` properly if used for slip casting.
Accordingly, it is the primary objective of this invention to provide a shell casting technique for slip casting molds that is 5 to 6 times more precise than the prior art with respect to linear change and flatness for slip casting molds.
It is another objective of the present invention to provide a shell casting method that facilitates automated slip-casting of intricate patterns.
It is yet another objective of the present invention to provide a technique for pressure casting of molten non-ferrous metals with its high surface density and useful for core molds with its ease of removal.